The present invention relates to implantable materials which can be formed into sheets of less than 0.8 mm for tissue reinforcement or into formed shapes for soft tissue augmentation. These materials have sufficient strength for the respective implantation applications. When implanted in a body the material promotes ingrowth of normal body tissue, and thereby becomes linked to adjacent tissue, and ultimately becomes filled with tissue. Thus, the materials may function in and of themselves or can stabilize appliances upon which they may be bonded; as, for example, to polymers for skeletal defect filling, to tendon replacements to which the materials are bonded at each end for securing the tendon replacement, to an implantable wear material for joint resurfacing applications or to other prosthetic joint implants for knee, hip and other skeletal joints.
U.S. Pat. No. 3,992,725 describes a composition of material suitable for in vivo implantation. It has a resilient, fibrous, porous structure; at least a portion of the biocompatible fibers of said structure have a critical surface tension above 35 mN/m on a scale extending from 20 to 80 mN/m. The structure includes biocompatible means for bonding said fibers into the porous structure having substantial interconnected spaces between the fibers. The structure possesses sufficient resiliency and distensibility when implanted to develop the type of ingrowth tissue needed at the implant location responsive to the mechanical forces at the implant site.
The typical formulation of said material was taught to be 80 vol % sodium chloride, 10 vol % carbon fibers, 6 vol % PTFE polymer fibers, and 4% PTFE particulate resin. The carbon fibers in the finished material (which is 80% porous because of the dissolving out of the sodium chloride component) provide the xe2x80x9cportion of the fibers of said structurexe2x80x9d having a surface tension greater than 35 mN/m. In effect the carbon fibers became very short during the manufacturing process, and became a dispersed particulate coating over a portion of the PTFE fibers. This formulation became widely used for surgical implantation.
Alternative fibrous materials to the carbon fibers are disclosed: Zirconia (zirconium oxide) fibers, Alumina (aluminum oxide) fibers, xe2x80x9cwhiskersxe2x80x9d of either alumina or zirconia, stainless steel wire.
The teachings and associated method of preparation of U.S. Pat. No. 3,992,725 yielded useful materials of 78-82% porosity with pore size range between 10 and 600 xcexcm (at col. 2, lines 58-60 of the patent). This pore size distribution resulted from the use of the fugitive salt ingredient having particle diameters between 10 and 600 xcexcm. These materials exhibited bulk properties of a stiff sponge in thicknesses greater than a few mm. Their properties allowed them to become widely used for augmenting the bony profile of the facial skeleton. In sheet form the products did not have strength useful for implantation applications at thickness less than about 0.8 mm; the stiff sponge-like bulk properties prevented use in direct soft tissue augmentation such as for cheek area and post-mastectomy breast reconstruction.
An object of the present invention is to provide an improved implantable composition of material which not only promotes ingrowth of normal body tissue, but has also an excellent tensile strength if a sheeting of a thickness of less than 0.8 mm, e.g. about 0.5 mm, is formed and/or, if formed into a monolithic shape, has the bulk properties of a soft sponge.
Further required properties of such a composition are:
suitability for stabilizing prostheses of biocompatible substances such as metal, polymers or elastomers
suitability in soft tissue augmentation
suitability for use in orthopedic rebuilding of joints and parts thereof
heat resistance, so that it can be sterilized by the usual steam autoclave procedures without being adversely effected thereby
suitability for implantation in a joint in that it is highly wear resistant and has low friction properties
capability of promoting ingrowth of tissue and being not sequestered by fibrous tissue when implanted.
Still a further object is to provide an improved method of making such an implantable material.
Many years of experimentation showed that about 82% porosity was the upper limit possible to prepare materials of useful mechanical integrity. Although sheeting from the prior art material as thin as 0.6 mm had been made, it was typically very fragile and therefore not commercially interesting.
Now it has been unexpectedly discovered that useful higher porosity (87%) and much softer products for soft tissue replacement, on the one hand, and about 0.4-0.5 mm high strength sheeting for tissue reinforcement, on the other, could be prepared if the particle size of the fugitive ingredient, salt, is selected to be narrow, in the range 150 to 300 xcexcm. This yields the equivalent narrow pore size distribution in the products. The ultimate tensile strength of 1 mm thick sheeting is increased from about 300 N/cm2 to 600 N/cm2 by reducing the pore size range to 150-300 xcexcm from 10-600 xcexcm.
It has also been surprisingly found that further enhancement of the tensile strength of the very thin 0.4-0.5 mm sheeting can be achieved by adding a second sintering step after the desired thickness of the sheeting has been obtained by a sanding process. These extra sanding and sintering steps would follow the sintering step (f) of the U.S. Pat. No. 3,992,725. The strength of such sheeting is about 300 N/cm2. This means that the tensile strength is doubled in relation to prior art products.
Thus, the inventor of the present invention has found that greatly improved product properties have resulted from two unexpectedly simple modifications to the process:
(A) a relatively narrow pore size distribution in the range of 150-300 xcexcm; and
(B) an additional sintering step when preparing sheeting at a thickness less than about 0.8 mm.
These and other objects and advantages are hereinafter set forth and explained.